Optical beam scanning device, image forming apparatus

ABSTRACT

An optical beam scanning device includes a polygon mirror  80  in which tilt angles with respect to a rotation axis of the polygon mirror  80  of respective plural reflecting surfaces are set to angles corresponding to photoconductive members associated with the respective reflecting surfaces, a post-deflection optical system A 1  that guides light beams reflected and deflected by the respective plural reflecting surfaces in the polygon mirror  80  to the photoconductive surfaces of the photoconductive members corresponding to the respective reflecting surfaces, and a pre-deflection optical system  7   a  that shapes the light from the light source to be a light beam of a predetermined sectional shape and guides the light beam to the polygon mirror  80,  the pre-deflection optical system  7   a  guiding the light from the light source to the polygon mirror  80  through an optical path that passes, in the main scanning direction, on an optical axis of the post-deflection optical system A 1  and passes, in the sub-scanning direction, a position further apart from the optical axis of the post-deflection optical system A 1  than all light beams after reflection and deflection guided by the post-deflection optical system A 1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 12/822,534filed on Jun. 24, 2010, which is a Continuation of application Ser. No.11/773,497 filed on Jul. 5, 2007, the entire contents of both of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical beam scanning device thatcauses a light beam from a light source to scan in a main scanningdirection on photoconductive surfaces of photoconductive members, and,more particularly to a technique for realizing improvement of an opticalcharacteristic.

2. Description of the Related Art

Conventionally, there is known a technique for, in image formingapparatuses that perform image formation of electrostatic latent imagesand the like on photoconductive members according to irradiation oflight beams, setting plural reflecting surfaces in a rotating deflector,which performs scanning of light beams on plural photoconductivemembers, to have different tilt angles with respect to a rotation axisand causing the light beams to perform scanning of differentphotoconductive members for each of the reflecting surfaces having thedifferent tilt angles (see JP-A-2000-2846 and JP-A-11-218991).

In a pre-deflection optical system in the conventional technique, a rayis made incident from the outside of a scanning range of apost-deflection optical system. In the conventional optical beamscanning device having such as constitution, there is a problem in that,when it is attempted to adopt an overfilled optical system advantageousfor an increase in speed, it is difficult to keep main scanningdirection beam diameters (which affect uniformity of quantities oflight) uniform and an optical characteristic is not stabilized.

SUMMARY OF THE INVENTION

It is an object of an embodiment of the present invention to provide atechnique that can contribute to improvement of an opticalcharacteristic of an optical beam scanning device while realizing anincrease in scanning speed of a light beam.

In order to solve the problem, an optical beam scanning device accordingto an aspect of the invention is an optical beam scanning device that iscapable of causing light from a light source to scan in a main scanningdirection on photoconductive surfaces of respective pluralphotoconductive members, the optical beam scanning device characterizedby including a rotating deflector that reflects and deflects an incidentlight beam with plural reflecting surfaces arrayed in association withthe respective plural photoconductive members in a rotating direction tothereby cause the incident light beam to scan in the main scanningdirection, tilt angles with respect to a rotation axis of the rotatingdeflector of the respective plural reflecting surfaces being set toangles corresponding to the photoconductive members associated with therespective reflecting surfaces, a post-deflection optical system thatguides light beams reflected and deflected by the respective pluralreflecting surfaces in the rotating deflector to the photoconductivesurfaces of the photoconductive members corresponding to the respectivereflecting surfaces, and a pre-deflection optical system that shapes thelight from the light source to be a light beam of a predeterminedsectional shape and guides the light beam to the rotating deflector, thepre-deflection optical system guiding the light from the light source tothe rotating deflector through an optical path that passes, in the mainscanning direction, on an optical axis of the post-deflection opticalsystem or near the optical axis and passes, in the sub-scanningdirection, a position further apart from the optical axis of thepost-deflection optical system than all light beams after reflection anddeflection guided by the post-deflection optical system.

An optical beam scanning device according to another aspect of theinvention is an optical beam scanning device that is capable of causinglight from a light source to scan in a main scanning direction onphotoconductive surfaces of respective plural photoconductive members,the optical beam scanning device characterized by including light beamdeflecting means for reflecting and deflecting an incident light beamwith plural reflecting surfaces arrayed in association with therespective plural photoconductive members in a rotating direction tothereby cause the incident light beam to scan in the main scanningdirection, tilt angles with respect to a rotation axis of the light beamdeflecting means of the respective plural reflecting surfaces being setto angles corresponding to the photoconductive members associated withthe respective reflecting surfaces, post-deflection light guiding meansfor guiding light beams reflected and deflected by the respective pluralreflecting surfaces in the light beam deflecting means to thephotoconductive surfaces of the photoconductive members corresponding tothe respective reflecting surfaces, and pre-deflection light guidingmeans for shaping the light from the light source to be a light beam ofa predetermined sectional shape and guiding the light beam to the lightbeam deflecting means, the pre-deflection light guiding means guidingthe light from the light source to the light beam deflecting meansthrough an optical path that passes, in the main scanning direction, onan optical axis of the post-deflection light guiding means or near theoptical axis and passes, in the sub-scanning direction, a positionfurther apart from the optical axis of the post-deflection light guidingmeans than all light beams after reflection and deflection guided by thepost-deflection light guiding means.

An image forming apparatus according to still another aspect of theinvention is characterized by including the optical beam scanning devicehaving the structure described above, a photoconductive member on whichan electrostatic latent image is formed by a light beam caused to scanby the optical beam scanning device, and a developing unit thatvisualizes the electrostatic latent image formed on the photoconductivemember.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a structure of an optical system of anoptical beam scanning device according to a first embodiment of theinvention in a state in which folding by a fold mirror is developed;

FIG. 2 is a sectional view in a sub-scanning direction showing aschematic structure of an image forming apparatus 900 including theoptical beam scanning device according to the first embodiment of theinvention;

FIG. 3 is a plan view showing a structure of an optical system of anoptical beam scanning device according to a second embodiment of theinvention in a state in which folding by a fold mirror is developed;

FIG. 4 is a plan view showing a structure of an optical system of anoptical beam scanning device according to a third embodiment of theinvention in a state in which folding by a fold mirror is developed;

FIG. 5 is a plan view showing a structure of an optical system of anoptical beam scanning device according to a fourth embodiment of theinvention in a state in which folding by a fold mirror is developed;

FIG. 6 is a plan view showing a structure of an optical system of anoptical beam scanning device according to a fifth embodiment of theinvention in a state in which folding by a fold mirror is developed;

FIG. 7 is a plan view showing a structure of an optical system of anoptical beam scanning device according to a sixth embodiment of theinvention in a state in which folding by a fold mirror is developed;

FIG. 8 is a plan view showing a structure of an optical system of anoptical beam scanning device according to a seventh embodiment of theinvention in a state in which folding by a fold mirror is developed;

FIG. 9 is a plan view showing a structure of an optical system of anoptical beam scanning device according to an eighth embodiment of theinvention in a state in which folding by a fold mirror is developed;

FIG. 10 is a plan view showing a structure of an optical system of anoptical beam scanning device according to a ninth embodiment of theinvention in a state in which folding by a fold mirror is developed;

FIG. 11 is a plan view showing a structure of an optical system of anoptical beam scanning device according to a tenth embodiment of theinvention in a state in which folding by a fold mirror is developed; and

FIG. 12 is a plan view showing a structure of an optical system of anoptical beam scanning device according to an eleventh embodiment of theinvention in a state in which folding by a fold mirror is developed.

DESCRIPTION OF THE EMBODIMENTS First Embodiment

A first embodiment of the present invention will be hereinafterexplained with reference to the drawings.

FIG. 1 is a plan view showing a structure of an optical system of anoptical beam scanning device according to the first embodiment of theinvention in a state in which folding by a fold mirror is developed.FIG. 2 is a sectional view in a sub-scanning direction showing aschematic structure of an image forming apparatus 900 including theoptical beam scanning device according to the first embodiment of theinvention.

As shown in FIGS. 1 and 2, the optical beam scanning device according tothis embodiment includes a pre-deflection optical system (pre-deflectionlight guiding means) 7 a, a polygon mirror (a rotating deflector, lightbeam deflecting means) 80, and a post-deflection optical system(post-deflection light guiding means) A1.

The optical beam scanning device has a role of causing a light beam froma light source 71 to scan in a main scanning direction onphotoconductive surfaces of respective plural photoconductive members401 y to 401 k. Electrostatic latent images are formed on thephotoconductive surfaces of the photoconductive members 401 y to 401 kby the light beam caused to scan by the optical beam scanning device.The electrostatic latent images formed on the respective photoconductivemembers are visualized by developing units 501 y to 501 k usingdeveloping agents of colors corresponding to the respectivephotoconductive members.

Details of the optical beam scanning device according to this embodimentwill be explained.

The polygon mirror 80 reflects and deflects an incident light beam withplural reflecting surfaces 80 y to 80 k arrayed in association with therespective plural photoconductive members 401 y to 401 k in a rotatingdirection to thereby cause the incident light beam to scan in the mainscanning direction. Tilt angles with respect to a rotation axis of thepolygon mirror 80 of the respective plural reflecting surfaces 80 y to80 k of the polygon mirror 80 are set to angles corresponding to thephotoconductive members associated with the respective reflectingsurfaces. In such a structure, the number of reflecting surfaces of thepolygon mirror 80 is integer times as large as the number of colors.Here, since four colors of yellow (401 y), magenta (401 m), cyan (401c), and black (401 k) are used, the number of reflecting surfaces of thepolygon mirror 80 is a multiple of 4 (4, 8, 12, . . . )

The pre-deflection optical system 7 a includes a light source 71 made ofan LD array having four light sources arranged in positions differentfrom one another in a sub-scanning direction (a rotation axis directionof the polygon mirror) orthogonal to the main scanning direction andcapable of blinking independently from one another, an infinite focuslens (or a collimator lens) 72 that changes a diverging ray from thelight source 71 to a convergent ray, a parallel ray, or a gentlediffused ray, and a cylinder lens 74, an fθ 1 lens 111, and an fθ 2 lens112 for condensing a light beam near the polygon mirror 80.

With such a structure, the pre-deflection optical system 7 a shapeslight from the light source 71 to be, for example, a light beam of apredetermined sectional shape long in the main scanning direction toguide the light beam to the polygon mirror 80 and condenses the lightbeam in the sub-scanning direction near the reflecting surfaces of thepolygon mirror 80.

The post-deflection optical system A1 is formed of a resin material suchas plastics and includes the fθ 1 lens 111 and the fθ 2 lens 112 thathave free-form surfaces of a power distribution in which power changescontinuously and optical elements such as a fold mirror shown in FIG. 2.In this way, in this embodiment, the fθ 1 lens 111 and the fθ 2 lens 112are used in both the optical systems of the pre-deflection opticalsystem 7 a and the post-deflection optical system A1 and give power toboth a light beam guided by the pre-deflection optical system 7 a and alight beam guided by the post-deflection optical system A1.

If the fθ 1 lens 111 and the fθ 2 lens 112 are not used in thepre-deflection optical system, when a light beam is guided from thelight source to the polygon mirror 80 by the pre-deflection opticalsystem, the light beam needs to pass an optical path that avoids the fθ1 lens 111 and the fθ 2 lens 112. However, in order to realize theoptical path that avoids the fθ 1 lens 111 and the fθ 2 lens 112described above, it is necessary to make the light beam incident on thepolygon mirror 80 at an angle substantially tilted with respect to anoptical axis. This is not preferable when occurrence of a waveaberration is taken into account.

On the other hand, as in this embodiment, if the fθ 1 lens 111 and thefθ 2 lens 112 are used in the pre-deflection optical system as well,when a light beam is guided from the light source to the polygon mirror80, it is possible to make the light beam incident on a deflectionsurface at an angle closer to the optical axis. Thus, it is possible tocontribute to control a wave aberration, a reduction in a device size inthe sub-scanning direction, and a reduction in the number of components.

With such a structure, the post-deflection optical system A1 guideslight beams reflected and deflected by the respective plural reflectingsurfaces 80 y to 80 k in the polygon mirror 80 to the photoconductivesurfaces of the photoconductive members 401 y to 401 k corresponding tothe respective reflecting surfaces through optical paths different fromone another. The post-deflection optical system A1 in this context isconstituted to guide, for example, after principal rays of light beamsat both ends in the sub-scanning direction among plural light beamsguided by the post-deflection optical system A1 pass the fθ 1 lens 111and the fθ 2 lens 112, light beams reflected and deflected by therespective plural reflecting surfaces in the polygon mirror 80 to thephotoconductive surfaces of the photoconductive members corresponding tothe respective reflecting surfaces through optical paths that pass anupper side and a lower side of an optical axis of the fθ 1 lens 111 andthe fθ 2 lens 112 in the sub-scanning direction.

In this embodiment, since the polygon mirror 80 has eight reflectingsurfaces, when one light beam is made incident on the polygon mirror, itis possible to write color information of four colors in the respectivephotoconductive members twice as the polygon mirror 80 rotates once.Here, a so-called “multi-beam optical system” in which the light source71 emits four light beams for forming electrostatic latent images on thephotoconductive surfaces independently from one another is adopted.Thus, it is possible to write color information of four colors for eightlines (four lines×2) at a time in the respective photoconductive membersas the polygon mirror 80 rotates once.

Since light sources combined into one array are used for image formationprocessing of primary colors (black, cyan, magenta, and yellow)corresponding to the respective photoconductive members as well, it ispossible to reduce the number of optical components, realize a reductionin cost, and realize a reduction in an arrangement space. When one lightsource is provided for each of the photoconductive members, in order toform latent images for four colors, it is necessary to increase thenumber of revolutions of a polygon mirror and a driving frequency of anLD by fourfold and it is difficult to increase speed of the imageformation processing and realize high definition of an image. However,in this embodiment, since the multi-beam optical system is adopted, itis possible to increase speed of formation of an electrostatic latentimage on a photoconductive drum without excessively increasing thenumber of revolutions of the polygon mirror and the driving frequency ofthe LD. Compared with a case in which plural light sources such as LDsare arranged in positions different from one another, it is possible toprevent occurrence of adjustment errors and the like of the arrangementpositions of the light sources and contribute to improvement of anoptical characteristic.

Curvatures of the fθ 1 lens 111 and the fθ 2 lens 112 independentlychange in two directions of the main scanning direction and thesub-scanning direction. The fθ 1 lens 111 and the fθ 2 lens 112 in thiscontext are equivalent to a shared optical element. Power distributionsof the respective fθ 1 lens 111 and fθ 2 lens 112 are set as powerdistributions that give power to all light beams reflected and deflectedby the polygon mirror 80 and guided to the respective pluralphotoconductive members 401 y to 401 k (all light beams reflected anddeflected by the respective plural reflecting surfaces) according topositions of incidence of the light beams such that the light beamsguided to the photoconductive surfaces by the post-deflection opticalsystem A1 have a predetermined optical characteristic (e.g., acharacteristic that satisfies predetermined conditions concerning a beamdiameter of a light beam, a degree of bending of a scanning line, aposition of the light beam with respect to a scanning range, and thelike) on the photoconductive surfaces. In this way, the shared opticalelement has a smooth lens surface that acts on all the light beamsreflected and deflected by the respective plural reflecting surfaces inthe polygon mirror 80.

As described above, by combining a part of the optical elementsconventionally provided independently for each of the photoconductivemembers into the shared optical element and giving power to all thelight beams, which should be guided to the plural photoconductivemembers, with the shared optical element, it is possible to contributeto a reduction in an arrangement space of optical components in thesub-scanning direction. Since it is possible to reduce the number ofoptical components that should be arranged, it is possible to preventdeterioration in an optical characteristic due to arrangement errors andthe like of the respective optical components and also contribute to areduction in cost.

By combining a part of the optical elements independently provided foreach of the photoconductive members into the shared optical system, itis possible to set tilt angles of the respective reflecting surfaces ofthe polygon mirror to small angles and reduce an arrangement space inthe sub-scanning direction of the optical system. Further, it ispossible to control occurrence of an asymmetrical wave aberration thatincreases when the tilt angles of the reflecting surfaces of the polygonmirror are large and realize improvement of a focusing characteristic aswell. Moreover, by applying the optical beam scanning device of such astructure to an image forming apparatus, it is possible to contribute toa reduction in size of the image forming apparatus and stabilization ofan image quality in image formation processing.

The “predetermined optical characteristic” in this context means anoptical characteristic desirable in forming electrostatic latent imageson the photoconductive surfaces of the photoconductive member. Bycondensing an incident light beam from the pre-deflection optical systemto the polygon mirror near the reflecting surfaces (setting the incidentlight in a conjugate relation in the sub-scanning direction on thereflecting surfaces of the polygon mirror and on the photoconductivesurfaces of the photoconductive members), a shift of a beam position inthe sub-scanning direction due to tilts of the respective reflectingsurfaces of the polygon mirror is controlled (surface topplingcorrection).

In this embodiment, the shared optical element is formed by the two fθlenses. However, the invention is not limited to this. The sharedoptical element may be formed by, for example, three or more lenses. Byforming the shared optical element with plural lenses in this way,compared with the case in which the shared optical element is formed byone lens, it is possible to set curvatures of lens surfaces of therespective lenses gentle, machining becomes easy, and it is possible tocontribute to a reduction in manufacturing cost and improvement ofmachining accuracy.

In this embodiment, a continuously changing power distribution is setfor both planes of incidence and planes of exit of the respective fθ 1lens and fθ 2 lens. However, it is not always necessary to set such apower distribution for all the lens surfaces of the shared opticalelement. In general, when the shared optical element is formed by theplural lenses in this way, lenses located on a downstream side in alight beam traveling direction often have larger sizes. In other words,since light beams made incident on the lenses on the downstream side inthe light beam traveling direction have smaller beam diameters and havelarge moving distances of the light beams at the same oscillating anglecompared with lenses located on an upstream side, it is considered thatthere is a significant effect in setting the continuously changing powerdistribution as described above. Thus, when the shared optical elementdescribed above is formed by the plural lenses, it is preferable to givecontinuously changing power described above to the side of the plane ofexit of the lens located on the most downstream side in the light beamtraveling direction (i.e., a side closest to an image surface).

It goes without saying that it is also possible to combine the fθ 1 lens111 and the fθ 2 lens 112 into one fθ lens (shared optical element).Consequently, it is possible to reduce the number of components of theoptical system and contribute to a reduction in cost compared with thestructure in which the two fθ lenses are used.

A structure of the entire optical system in the optical beam scanningdevice according to this embodiment and a relation between thepre-deflection optical system 7 a and the post-deflection optical systemA1 will be explained in detail.

The optical beam scanning device according to this embodimentconstitutes a so-called “overfilled optical system”. The “overfilledoptical system” is an optical system that makes a light beam wider thanthe respective reflecting surfaces of the polygon mirror incident on thepolygon mirror and uses the reflecting surfaces as substantial apertures(stops) in the main scanning direction. By adopting such an “overfilledoptical system”, it is possible to increase the number of surfaces ofthe polygon mirror without increasing a size thereof and contribute toan increase in speed compared with an “underfilled optical system” thatmakes a light beam narrower than the reflecting surfaces of the polygonmirror incident and deflects and causes the light beam to scan.

In this embodiment, in order to control variation in a beam diameter anda quantity of light, which is a problem in adopting the overfilledoptical system, as much as possible, a ray is made incident on thepolygon mirror from a front direction thereof.

The pre-deflection optical system 7 a in this embodiment guides lightfrom the light source 71 to the polygon mirror 80 through an opticalpath that passes, in the main scanning direction, on an optical axis ofthe post-deflection optical system A1 (or near the optical axis) andpasses (see FIG. 1), in the sub-scanning direction, a position furtherapart from the optical axis of the post-deflection optical system A1than all light beams after reflection and deflection guided by thepost-deflection optical system A1 (see FIG. 2).

In the pre-deflection optical system 7 a, as shown in FIG. 2, theoptical path for guiding a light beam from the light source 71 to thepolygon mirror 80 is set to be apart from the fold mirror (see FIG. 2),which is an optical element arranged in a position closest to the lightsource 71 among the optical elements forming the post-reflection opticalsystem A1, by a distance twice or more as large as a diameter of thelight beam near the fold mirror.

In such a structure, light emitted from the light source 71 is guided tothe reflecting surfaces of the polygon mirror 80 through the finitefocus lens 72, the cylinder lens 74, the fθ 1 lens 111, and the fθ 2lens 112. A light beam reflected and deflected by the reflectingsurfaces of the polygon mirror 80 is caused to scan toward thephotoconductive surfaces of the photoconductive member through the fθ 1lens 111 and the fθ 2 lens 112 again.

Second Embodiment

A second embodiment of the invention will be explained.

FIG. 3 is a plan view showing a structure of an optical system of anoptical beam scanning device according to the second embodiment of theinvention in a state in which folding by a fold mirror is developed.

In this embodiment, a diffraction grating is formed on a first surface112 k of the fθ 2 lens 112 among plural optical elements forming apost-deflection optical system A2. The fθ 2 lens 112 in this context isan optical element in which respective light beams, which should beguided to the respective photoconductive members 401 y to 401 k, aremade incident on positions of incidence different from one another inthe sub-scanning direction.

In a semiconductor laser, when an environmental temperature fluctuates,a wavelength of light emitted by the semiconductor laser also changesaccording to this temperature change. The diffraction grating has acharacteristic that power also changes according to fluctuation in awavelength of an incident light and an environmental temperature.

In general, only correction corresponding to a temperature change ispossible by simply arranging an optical element without a diffractiongrating and a range in which light beams can be corrected is limited.Thus, in this embodiment, the diffraction grating is provided in the fθ2 lens 112 to widen the range in which light beams can be corrected. Itis possible to perform only correction corresponding to “temperature”with the lens alone. However, when the diffraction grating is added, itis also possible to perform correction corresponding to “wavelength”.This makes it possible to increase adjustment parameters and improve adegree of freedom of correction.

By adopting such a structure, in addition to the effects realized by thestructure in the first embodiment, it is possible to correct fluctuationin an optical characteristic of the scanning optical system due to atemperature change. Further, since the diffraction grating is formed inthe existing optical element, the number of components of the opticalelement is not increased.

The first surface 112 k of the fθ 2 lens 112 is equivalent to a plane ofincidence of a light beam guided by the pre-deflection optical system 7a and is equivalent to a plane of exit of a light beam guided by thepost-deflection optical system 7 a.

Third Embodiment

A third embodiment of the invention will be explained.

This embodiment is a modification of the second embodiment.

FIG. 4 is a plan view showing a structure of an optical system of anoptical beam scanning device according to the third embodiment of theinvention in a state in which folding by a fold mirror is developed.

In this embodiment, a diffraction grating is formed on a second surface112 f of the fθ 2 lens 112 among plural optical elements forming apost-deflection optical system A3.

Fourth Embodiment

A fourth embodiment of the invention will be explained.

FIG. 5 is a plan view showing a structure of an optical system of anoptical beam scanning device according to the fourth embodiment of theinvention in a state in which folding by a fold mirror is developed.

In this embodiment, a diffraction grating is formed on a first surface111 k of the fθ 1 lens 111 among plural optical elements forming apost-deflection optical system A4. The fθ 1 lens 111 in this context isan optical element in which respective light beams, which should beguided to the respective photoconductive members 401 y to 401 k, aremade incident on positions of incidence different from one another inthe sub-scanning direction.

The first surface 111 k of the fθ 1 lens 111 is equivalent to a plane ofincidence of a light beam guided by the pre-deflection optical system 7a and is equivalent to a plane of exit of a light beam guided by thepost-deflection optical system 7 a.

Fifth Embodiment

A fifth embodiment of the invention will be explained.

This embodiment is a modification of the fourth embodiment.

FIG. 6 is a plan view showing a structure of an optical system of anoptical beam scanning device according to the fifth embodiment of theinvention in a state in which folding by a fold mirror is developed.

In this embodiment, a diffraction grating is formed on a second surface111 f of the fθ 1 lens 111 among plural optical elements forming apost-deflection optical system A5.

Sixth Embodiment

A sixth embodiment of the invention will be explained.

FIG. 7 is a plan view showing a structure of an optical system of anoptical beam scanning device according to the sixth embodiment of theinvention in a state in which folding by a fold mirror is developed.

In this embodiment, a post-deflection optical system A6 has a tabularoptical element 130, in which a diffraction grating is formed on a firstsurface 130 k, between the fθ 2 lens 112 and the light source 71 as anoptical element having a diffraction grating formed therein.

By arranging the tabular optical element having only power by thediffraction grating in the post-deflection optical system in this way,it is possible to adjust a distance between the diffraction grating andthe optical element adjacent to the diffraction grating. Thus, it ispossible to realize a more excellent optical characteristic comparedwith the case in which the diffraction grating is formed in the existingoptical element.

Seventh Embodiment

A seventh embodiment of the invention will be explained.

This embodiment is a modification of the sixth embodiment.

FIG. 8 is a plan view showing a structure of an optical system of anoptical beam scanning device according to the seventh embodiment of theinvention in a state in which folding by a fold mirror is developed.

In this embodiment, a post-deflection optical system A7 has the tabularoptical element 130, in which a diffraction grating is formed on asecond surface 130 f, between the fθ 2 lens 112 and the light source 71as an optical element having a diffraction grating formed therein.

Eighth Embodiment

An eighth embodiment of the invention will be explained.

FIG. 9 is a plan view showing a structure of an optical system of anoptical beam scanning device according to the eighth embodiment of theinvention in a state in which folding by a fold mirror is developed.

In this embodiment, a post-deflection optical system A8 has the tabularoptical element 130, in which a diffraction grating is formed on thefirst surface 130 k, between the fθ 1 lens 111 and the fθ 2 lens 112 asan optical element having a diffraction grating formed therein.

By arranging the tabular optical element having only power by thediffraction grating in the post-deflection optical system in this way,it is possible to adjust a distance between the diffraction grating andthe optical element adjacent to the diffraction grating. Thus, it ispossible to realize a more excellent optical characteristic comparedwith the case in which the diffraction grating is formed in the existingoptical element.

Ninth Embodiment

A ninth embodiment of the invention will be explained.

This embodiment is a modification of the eighth embodiment.

FIG. 10 is a plan view showing a structure of an optical system of anoptical beam scanning device according to the ninth embodiment of theinvention in a state in which folding by a fold mirror is developed.

In this embodiment, a post-deflection optical system A9 has the tabularoptical element 130, in which a diffraction grating is formed on thesecond surface 130 f, between the fθ 1 lens 111 and the fθ 2 lens 112 asan optical element having a diffraction grating formed therein.

Tenth Embodiment

A tenth embodiment of the invention will be explained.

FIG. 11 is a plan view showing a structure of an optical system of anoptical beam scanning device according to the tenth embodiment of theinvention in a state in which folding by a fold mirror is developed.

In this embodiment, a post-deflection optical system A10 has the tabularoptical element 130, in which a diffraction grating is formed on thefirst surface 130 k, between the fθ 1 lens 111 and the polygon mirror 80as an optical element having a diffraction grating formed therein.

By arranging the tabular optical element having only power by thediffraction grating in the post-deflection optical system in this way,it is possible to adjust a distance between the diffraction grating andthe optical element adjacent to the diffraction grating. Thus, it ispossible to realize a more excellent optical characteristic comparedwith the case in which the diffraction grating is formed in the existingoptical element.

Eleventh Embodiment

An eleventh embodiment of the invention will be explained.

This embodiment is a modification of the tenth embodiment.

FIG. 12 is a plan view showing a structure of an optical system of anoptical beam scanning device according to the eleventh embodiment of theinvention in a state in which folding by a fold mirror is developed.

In this embodiment, a post-deflection optical system A11 has the tabularoptical element 130, in which a diffraction grating is formed on thesecond surface 130 f, between the fθ 1 lens 111 and the polygon mirror80 as an optical element having a diffraction grating formed therein.

In the respective embodiments, the structure in which both the lightbeam guided by the pre-deflection optical system and the light beamguided by the post-deflection optical system pass through the fθ 1 lens111 and the fθ 2 lens 112 is described as the example. However, theinvention is not limited to this. At least one of the light beam guidedby the pre-deflection optical system and the light beam guided by thepost-deflection optical system may pass through at least one of the fθ 1lens 111 and the fθ 2 lens 112.

Concerning the optical element (e.g., the tabular optical element 130)in which the diffraction grating is formed, similarly, the structure inwhich both the light beam guided by the pre-deflection optical systemand the light beam guided by the post-deflection optical system passthrough the diffraction grating is described as an example. However, theinvention is not limited to this. At least one of the light beam guidedby the pre-deflection optical system and the light beam guided by thepost-deflection optical system may pass through the diffraction grating.

In the respective embodiments, the structure in which one opticalelement having the diffraction grating formed therein is arranged forone optical path of a light beam is described as an example. However,the invention is not limited to this. For example, two optical elementshaving diffraction gratings formed therein are arranged on an opticalpath and a diffraction grating having power in the main scanningdirection and a diffraction grating having power in the sub-scanningdirection are separately formed in the two optical elements. This makesit possible to improve a degree of freedom of adjustment by thediffraction gratings and contribute to improvement of opticalperformance as well.

In the respective embodiments, the example in which the “multi-beamoptical system” is adopted as the light source 71 is described. However,the invention is not limited to this. The invention is also effective ina structure in which an optical system that emits only one light beamfrom the light source 71 is adopted.

In the respective embodiments, the structure in which the tabularoptical element (the optical element having only power by thediffraction grating) having the diffraction grating formed therein isadded to the existing optical system. However, the invention is notlimited to this. It goes without saying that it is also possible toadopt a structure in which a lens having negative or positive power andhaving a diffraction grating formed therein is added to an opticalsystem.

As described above, according to the respective embodiments, it ispossible to reduce an angular difference in the sub-scanning directionof respective light beams emitted from the polygon mirror correspondingto the respective colors (Y, M, C, and K). Thus, there is an effect thatit is possible to reduce an effective angle in the sub-scanningdirection of the post-deflection optical system and it is easy to secureuniformity of image surface curves, fθ characteristics, and intervalsamong the respective colors (when the uniformity is lost, a color driftis caused).

The invention has been explained in detail according to the specificforms. However, it would be obvious for those skilled in the art thatvarious modifications and alterations could be made without departingfrom the spirit and the scope of the invention.

As described in detail above, according to the invention, it is possibleto provide a technique that can contribute to improvement of an opticalcharacteristic of the optical beam scanning device while realizing anincrease in scanning speed of a light beam.

1. An optical beam scanning device that is capable of causing light froma light source to scan in a main scanning direction on photoconductivesurfaces of respective plural photoconductive members, the optical beamscanning device comprising: a rotating deflector that reflects anddeflects an incident light beam with plural reflecting surfaces arrayedin association with the respective plural photoconductive members in arotating direction to thereby cause the incident light beam to scan inthe main scanning direction, tilt angles with respect to a rotation axisof the rotating deflector of the respective plural reflecting surfacesbeing set to angles corresponding to the photoconductive membersassociated with the respective reflecting surfaces; a post-deflectionoptical system that guides light beams reflected and deflected by therespective plural reflecting surfaces in the rotating deflector to thephotoconductive surfaces of the photoconductive members corresponding tothe respective reflecting surfaces; and a pre-deflection optical systemthat shapes the light from the light source to be a light beam of apredetermined sectional shape and guides the light beam to the rotatingdeflector, the pre-deflection optical system guiding the light from thelight source to the rotating deflector through an optical path thatpasses, in the sub-scanning direction, a position further apart from theoptical axis of the post-deflection optical system than all light beamsafter reflection and deflection guided by the post-deflection opticalsystem, the optical beam scanning device being an overfilled opticalsystem.
 2. The optical beam scanning device according to claim 1, thepre-deflection optical system guides the light from the light source tothe rotating deflector through an optical path that passes, in the mainscanning direction, on an optical axis of the post-deflection opticalsystem or near the optical axis.
 3. The optical beam scanning deviceaccording to claim 1, in the pre-deflection optical system, an opticalpath for guiding a light beam from the light source to the rotatingdeflector is set to be apart from an optical element, which is arrangedin a position closest to the light source among optical elements formingthe post-reflection optical system, by a distance twice or more as largeas a diameter of the light beam.
 4. The optical beam scanning deviceaccording to claim 1, among plural optical elements forming thepost-deflection optical system, in at least one optical element in whichrespective light beams, which should be guided to the respective pluralphotoconductive members, are made incident on positions of incidencedifferent from one another in the sub-scanning direction, a diffractiongrating is formed on at least one of a plane of incidence and a plane ofexit of the light beams in the optical element.
 5. The optical beamscanning device according to claim 4, the post-deflection optical systemhas, as the optical element having the diffraction grating formedtherein, a tabular optical element in which a grating is formed on atleast one of a plane of incidence and a plane of exit.
 6. The opticalbeam scanning device according to claim 1, the pre-deflection opticalsystem shapes the light from the light source to be a light beam of apredetermined sectional shape and guides the light beam to the rotatingdeflector and condenses the light beam in the sub-scanning directionnear the reflecting surfaces of the rotating deflector.
 7. An opticalbeam scanning device that is capable of causing light from a lightsource to scan in a main scanning direction on photoconductive surfacesof respective plural photoconductive members, the optical beam scanningdevice comprising: light beam deflecting means for reflecting anddeflecting an incident light beam with plural reflecting surfacesarrayed in association with the respective plural photoconductivemembers in a rotating direction to thereby cause the incident light beamto scan in the main scanning direction, tilt angles with respect to arotation axis of the light beam deflecting means of the respectiveplural reflecting surfaces being set to angles corresponding to thephotoconductive members associated with the respective reflectingsurfaces; post-deflection light guiding means for guiding light beamsreflected and deflected by the respective plural reflecting surfaces inthe light beam deflecting means to the photoconductive surfaces of thephotoconductive members corresponding to the respective reflectingsurfaces; the post-deflection light guiding means includes a sharedoptical element that gives, according to positions of incidence of thelight beams, power to light beams, which should be reflected anddeflected by the light beam deflecting means and guided to therespective plural photoconductive members, such that light beams guidedto the photoconductive surfaces by the post-deflection light guidingmeans have a predetermined optical characteristic on the photoconductivesurfaces; the post-deflection light guiding means guides, afterprincipal rays of light beams located at both ends in the sub-scanningdirection among plural light beams guided by the post-deflection lightguiding means pass the shared optical element, light beams reflected anddeflected by the respective plural reflecting surfaces in the light beamdeflecting means to the photoconductive surfaces of the photoconductivemember corresponding to the respective reflecting surfaces throughoptical paths that pass an upper side and a lower side of an opticalaxis of the shared optical element in the sub-scanning direction; allthe respective light beams which should be reflected and deflected bythe light beam deflecting means and guided to the respective pluralphotoconductive members, pass the shared optical element; pre-deflectionlight guiding means for shaping the light from the light source to be alight beam of a predetermined sectional shape and guiding the light beamto the light beam deflecting means, the pre-deflection light guidingmeans guiding the light from the light source to the light beamdeflecting means through an optical path that passes, in thesub-scanning direction, a position further apart from the optical axisof the post-deflection light guiding means than all light beams afterreflection and deflection guided by the post-deflection light guidingmeans; and the optical beam scanning device being an overfilled opticalsystem.
 8. An optical beam scanning device according to claim 7, thepre-deflection light guiding means guides the light from the lightsource to the light beam deflecting means through an optical path thatpasses, in the main scanning direction, on an optical axis of thepost-deflection light guiding means or near the optical axis.
 9. Anoptical beam scanning device according to claim 7, in the pre-deflectionlight guiding means, an optical path for guiding a light beam from thelight source to the light beam deflecting means is set to be apart froman optical element, which is arranged in a position closest to the lightsource among optical elements forming the post-reflection light guidingmeans, by a distance twice or more as large as a diameter of the lightbeam.
 10. An optical beam scanning device according to claim 7, amongplural optical elements forming the post-deflection light guiding means,in at least one optical element in which respective light beams, whichshould be guided to the respective plural photoconductive members, aremade incident on positions of incidence different from one another inthe sub-scanning direction, a diffraction grating is formed on at leastone of a plane of incidence and a plane of exit of the light beams inthe optical element.
 11. An optical beam scanning device according toclaim 10, the post-deflection light guiding means has, as the opticalelement having the diffraction grating formed therein, a tabular opticalelement in which a diffraction grating is formed on at least one of aplane of incidence and a plane of exit.
 12. An optical beam scanningdevice according to claim 7, the pre-deflection light guiding meansshapes the light from the light source to be a light beam of apredetermined sectional shape and guides the light beam to the lightbeam deflecting means and condenses the light beam in the sub-scanningdirection near the reflecting surfaces of the light beam deflectingmeans.
 13. An optical beam scanning device that is capable of causinglight from a light source to scan in a main scanning direction onphotoconductive surfaces of respective plural photoconductive members,the optical beam scanning device comprising: light beam deflecting meansfor reflecting and deflecting an incident light beam with pluralreflecting surfaces arrayed in association with the respective pluralphotoconductive members in a rotating direction to thereby cause theincident light beam to scan in the main scanning direction, tilt angleswith respect to a rotation axis of the light beam deflecting means ofthe respective plural reflecting surfaces being set to anglescorresponding to the photoconductive members associated with therespective reflecting surfaces; post-deflection light guiding means forguiding light beams reflected and deflected by the respective pluralreflecting surfaces in the light beam deflecting means to thephotoconductive surfaces of the photoconductive members corresponding tothe respective reflecting surfaces; pre-deflection light guiding meansfor shaping the light from the light source to be a light beam of apredetermined sectional shape and guiding the light beam to the lightbeam deflecting means, the pre-deflection light guiding means guidingthe light from the light source to the light beam deflecting meansthrough an optical path that passes, in the sub-scanning direction, aposition further apart from the optical axis of the post-deflectionlight guiding means than all light beams after reflection and deflectionguided by the post-deflection light guiding means; and the optical beamscanning device being an overfilled optical system.
 14. An optical beamscanning device according to claim 13, the pre-deflection light guidingmeans guides the light from the light source to the light beamdeflecting means through an optical path that passes, in the mainscanning direction, on an optical axis of the post-deflection lightguiding means or near the optical axis.
 15. An optical beam scanningdevice according to claim 13, in the pre-deflection light guiding means,an optical path for guiding a light beam from the light source to thelight beam deflecting means is set to be apart from an optical element,which is arranged in a position closest to the light source amongoptical elements forming the post-reflection light guiding means, by adistance twice or more as large as a diameter of the light beam.
 16. Anoptical beam scanning device according to claim 13, among plural opticalelements forming the post-deflection light guiding means, in at leastone optical element in which respective light beams, which should beguided to the respective plural photoconductive members, are madeincident on positions of incidence different from one another in thesub-scanning direction, a diffraction grating is formed on at least oneof a plane of incidence and a plane of exit of the light beams in theoptical element.
 17. An optical beam scanning device according to claim16, the post-deflection light guiding means has, as the optical elementhaving the diffraction grating formed therein, a tabular optical elementin which a diffraction grating is formed on at least one of a plane ofincidence and a plane of exit.
 18. An optical beam scanning deviceaccording to claim 13, the post-deflection light guiding means includesa shared optical element that gives, according to positions of incidenceof the light beams, power to light beams, which should be reflected anddeflected by the light beam deflecting means and guided to therespective plural photoconductive members, such that light beams guidedto the photoconductive surfaces by the post-deflection light guidingmeans have a predetermined optical characteristic on the photoconductivesurfaces.
 19. An optical beam scanning device according to claim 18, thepost-deflection light guiding means guides, after principal rays oflight beams located at both ends in the sub-scanning direction amongplural light beams guided by the post-deflection light guiding meanspass the shared optical element, light beams reflected and deflected bythe respective plural reflecting surfaces in the light beam deflectingmeans to the photoconductive surfaces of the photoconductive membercorresponding to the respective reflecting surfaces through opticalpaths that pass an upper side and a lower side of an optical axis of theshared optical element in the sub-scanning direction.
 20. An opticalbeam scanning device according to claim 13, the pre-deflection lightguiding means shapes the light from the light source to be a light beamof a predetermined sectional shape and guides the light beam to thelight beam deflecting means and condenses the light beam in thesub-scanning direction near the reflecting surfaces of the light beamdeflecting means.